U.S. patent application number 14/413876 was filed with the patent office on 2015-07-23 for fuel cell interconnector and method for making a fuel cell interconnector.
The applicant listed for this patent is STACKPOLE INTERNATIONAL POWDER METAL, ULC. Invention is credited to Brendan Ayre, Roger Lawcock, Rohith Shivanath.
Application Number | 20150207154 14/413876 |
Document ID | / |
Family ID | 49915469 |
Filed Date | 2015-07-23 |
United States Patent
Application |
20150207154 |
Kind Code |
A1 |
Shivanath; Rohith ; et
al. |
July 23, 2015 |
FUEL CELL INTERCONNECTOR AND METHOD FOR MAKING A FUEL CELL
INTERCONNECTOR
Abstract
An interconnector for a solid oxide fuel cell is manufactured by
single-press compacting a powder blend to form a green
interconnector with a desired shape of a final interconnector. The
powder blend includes chromium and iron, and may include an organic
lubricant. At least 50 wt % or more of an iron portion of the
powder blend comprises iron particles smaller than 45 um. The green
interconnector is then sintered and oxidized to form the final
interconnector. The oxidation step occurs in a continuous flow
furnace in which a controlled atmosphere (e.g., humidified air) is
fed into the furnace in the travel direction of the interconnector.
The final interconnector comprises at least 90 wt % chromium, at
least 3 wt % iron, and less than 0.2 wt % nitrogen. An average
density within a flow field of the final interconnector may be less
than 6.75 g/cc.
Inventors: |
Shivanath; Rohith; (Toronto,
CA) ; Ayre; Brendan; (Waterdown, CA) ;
Lawcock; Roger; (Burlington, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
STACKPOLE INTERNATIONAL POWDER METAL, ULC |
Ancaster |
|
CA |
|
|
Family ID: |
49915469 |
Appl. No.: |
14/413876 |
Filed: |
July 8, 2013 |
PCT Filed: |
July 8, 2013 |
PCT NO: |
PCT/IB2013/001476 |
371 Date: |
January 9, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61669537 |
Jul 9, 2012 |
|
|
|
61701956 |
Sep 17, 2012 |
|
|
|
Current U.S.
Class: |
429/514 ;
148/237; 419/2 |
Current CPC
Class: |
B22F 3/24 20130101; H01M
8/0245 20130101; C23C 8/10 20130101; H01M 8/0243 20130101; B22F
2003/242 20130101; B22F 3/12 20130101; C23C 8/24 20130101; Y02E
60/50 20130101; Y02P 70/50 20151101; H01M 2008/1293 20130101; C23C
8/02 20130101; B22F 1/0059 20130101; B22F 2999/00 20130101; C22C
1/045 20130101; B22F 1/0014 20130101; B22F 2998/10 20130101; H01M
8/0232 20130101; B22F 2998/10 20130101; B22F 1/0059 20130101; B22F
3/02 20130101; B22F 3/1021 20130101; B22F 3/10 20130101; B22F
2003/248 20130101; B22F 2999/00 20130101; B22F 2003/248 20130101;
B22F 2201/02 20130101; B22F 2201/03 20130101; B22F 2201/05
20130101 |
International
Class: |
H01M 8/02 20060101
H01M008/02; B22F 3/24 20060101 B22F003/24; B22F 1/00 20060101
B22F001/00; C23C 8/24 20060101 C23C008/24; B22F 3/12 20060101
B22F003/12 |
Claims
1. A method of oxidizing a porous component comprising at least 20
weight % chromium, the method comprising: oxidizing the component
in a furnace so as to expose the component to an oxidation
temperature range for a predetermined time period; and during said
oxidizing, feeding a controlled atmosphere into the furnace,
wherein the controlled atmosphere comprises: at least 30 volume %
nitrogen, at least 10 volume % oxygen, and at least 10 volume %
water vapor, wherein said oxidizing increases a nitrogen content of
the porous component by less than 0.1 weight %.
2. The method of claim 1, wherein after said oxidizing, the
component comprises less than 0.2 weight % nitrogen.
3. The method of claim 1, wherein after said oxidizing, the
component comprises less than 0.15 weight % nitrogen.
4. The method of claim 1, wherein the controlled atmosphere
comprises at least 50 volume % ambient air.
5. The method of claim 1, wherein the controlled atmosphere
comprises at least 20 volume % water vapor.
6. The method of claim 5, wherein the controlled atmosphere
comprises between 10 and 30 volume % water vapor.
7. The method of claim 1, further comprising: adding water vapor to
ambient air to create the controlled atmosphere.
8. The method of claim 1, wherein the oxidation temperature range
is above 750.degree. C. and the predetermined time period is at
least 5 hours.
9. The method of claim 1, further comprising feeding the component
through the furnace in a travel direction during said oxidizing,
wherein the controlled atmosphere is fed into the furnace in the
travel direction.
10. The method of claim 1, further comprising feeding the component
through the furnace in a travel direction during said oxidizing,
wherein the controlled atmosphere is fed into the furnace in an
opposite direction as the travel direction.
11. The method of claim 1, wherein the component comprises an SOFC
interconnector.
12. A method of manufacturing an interconnector for a solid oxide
fuel cell, the method comprising: single-press compacting a powder
blend to form a green interconnector with a desired shape of a
final interconnector, the powder blend comprising chromium and
iron, at least 50 wt % of an iron portion of the powder blend
comprising iron particles smaller than 45 um; and sintering the
single-pressed green interconnector to form a sintered
interconnector, wherein the sintered interconnector comprises at
least 90 wt % chromium and at least 3 wt % iron.
13. The method of claim 12, wherein at least 80 wt % of the iron
portion of the powder blend comprises iron particles smaller than
45 um.
14. The method of claim 13, wherein at least 90 wt % of the iron
portion of the powder blend comprises iron particles smaller than
45 um.
15. The method of claim 12, wherein at least 50 wt % of the iron
portion, of the powder blend comprises iron particles smaller than
20 um.
16. The method of claim 12, wherein the sintered interconnector
comprises between 94.5 and 95.5 wt % chromium and between 4.5 and
5.5 wt % iron.
17. The method of claim 12, further comprising: blending iron
powder and an organic lubricant to form a master iron/lubricant
blend, wherein the lubricant comprises at least 5 wt % of the
master iron/lubricant blend; blending the master iron/lubricant
blend with chromium powder to form the powder blend; and
delubricating the green interconnector before said sintering.
18. The method of claim 17, wherein the lubricant comprises at
least 10 wt % of the master iron/lubricant blend.
19. The method of claim 12, wherein said sintering occurs over a
sintering cycle time at a sintering temperature range that does not
exceed 1425.degree. C.
20. The method of claim 19, wherein said sintering temperature
range does not exceed 1400.degree. C.
21. The method of claim 19, wherein said sintering temperature
range does not fall below 1150.degree. C., and wherein the
sintering cycle time is less than 3 hours.
22. The method of claim 21, wherein the sintering cycle time is
less than 2 hours.
23. The method of claim 19, wherein said sintering results in at
least 70% diffusion of the chromium into the iron.
24. The method of claim 23, wherein said sintering results in at
least 80% diffusion of the chromium into the iron.
25. The method of claim 12, further comprising oxidizing the
sintered interconnector to form a final interconnector, wherein the
final interconnector is impermeable to air and SOFC fuel.
26. The method of claim 25, wherein said oxidizing comprises
passing the sintered interconnector through a continuous flow
furnace in an interconnector travel direction while feeding an
oxygen containing gas into the furnace in the interconnector travel
direction.
27. The method of claim 25, wherein said oxidizing comprises:
oxidizing the sintered interconnector in a furnace so as to expose
the sintered interconnector to an oxidation temperature range for a
predetermined time period; and during said oxidizing, feeding a
controlled atmosphere into the furnace, wherein the controlled
atmosphere comprises: at least 30 volume % nitrogen, at least 10
volume % oxygen, and at least 10 volume % water vapor, wherein
after said oxidizing, the final interconnector comprises less than
0.2 weight % nitrogen.
28. The method of claim 25, wherein: the final interconnector
comprises a flow field over which air or gas is designed to flow
during use of the interconnector; the flow field is impermeable to
SOFC fuel and air; and the final interconnector has an average
density within the flow field of less than 6.8 g/cc.
29. The method of claim 28, wherein the final interconnector has an
average density within the flow field of less than 6.75 g/cc.
30. The method of claim 12, wherein: the green interconnector
comprises a flow field over which air or gas is designed to flow
during use of the interconnector, and the green interconnector has
an average density within the flow field of less than 6.75
g/cc.
31. The method of claim 30, wherein the green interconnector has an
average density within the flow field of less than 6.70 g/cc.
32. An interconnector for a solid oxide fuel cell, the
interconnector comprising a sintered body comprising at least 90 wt
% chromium and at least 3 wt % iron, wherein the body defines a
flow field over which air or gas is designed to flow during use of
the interconnector, wherein an average density within the flow
field is less than 6.75 g/cc, and the flow field is impermeable to
SOFC fuel and air.
33. The interconnector of claim 32, wherein the interconnector is
formed from a pressed powder blend in which at least 50 wt % of an
iron portion of the powder blend comprised iron particles smaller
than 45 um.
34. The interconnector of claim 32, wherein the interconnector
comprises less than 0.2 weight % nitrogen.
35. An interconnector for a solid oxide fuel cell, the
interconnector comprising a sintered body comprising at least 90 wt
% chromium and at least 3 wt % iron, wherein the interconnector is
formed from a pressed powder blend in which at least 50 wt % of an
iron portion of the powder blend comprised iron particles smaller
than 45 um.
Description
CROSS-REFERENCE
[0001] This application claims priority to U.S. Provisional
Application No. 61/701,956, titled "FUEL CELL INTERCONNECTOR AND
METHOD FOR MAKING A FUEL CELL INTERCONNECTOR," filed Sep. 17, 2012,
and U.S. Provisional Application No. 61/669,537, titled "FUEL CELL
INTERCONNECTOR AND METHOD FOR MAKING A FUEL CELL INTERCONNECTOR,"
filed Jul. 9, 2012, the entire contents of both of which are hereby
incorporated by reference herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to interconnectors
for solid oxide fuel cells (SOFC), and methods of manufacturing
SOFC interconnectors using pressed powder metallurgy. Additionally
and/or alternatively, the present invention relates to the
controlled oxidation of porous chromium alloys such as
interconnectors for SOFCs.
[0004] 2. Description of Related Art
[0005] SOFCs directly produce electricity by oxidizing a fuel. In a
typical planar geometry SOFC, an electrolyte layer (solid oxide or
ceramic) is sandwiched between two electrodes (a cathode layer and
an anode layer). Fuel flows past the outside of the anode layer
(the oxidizing side) to provide H.sub.2 to the anode. Air flows
past the outside of the cathode layer (the reducing side) to
provide O.sub.2 to the cathode layer. The H.sub.2 and an O.sup.-
from the O.sub.2 react to produce H.sub.2O, which is exhausted on
the fuel side of the anode. The reaction causes electron flow from
the anode to the cathode, which provides electricity.
[0006] Individual SOFCs are typically stacked so that their
electrical output is combined in series. An interconnector (also
known as an interconnector plate or separator plate) separates
adjacent SOFCs. As a result, opposing sides of an interconnector
are exposed to the fuel side/oxidizing side of one SOFC and the air
side/reducing side of an adjacent SOFC. The interconnector is
typically designed to be substantially impermeable to the gaseous
phase air and fuel so as to minimize uncontrolled combustion and
catastrophic failure of an SOFC stack. An elevated temperature
oxidation process step is often used in the PM manufacturing
process whereby growth of an oxide layer is encouraged on the walls
of the internal porosity such that internal pore channels become
blocked by the formed oxide films and hence the oxidation process
provides a desirable reduction in permeability relative to the
un-oxidized condition.
[0007] End-plates are disposed at the end of an SOFC stack, and
function as one-sided interconnectors. For ease of reference, an
end plate is defined herein to be an interconnector.
[0008] Typical operating temperatures of SOFCs are between
600.degree. C. and 1000.degree. C.
[0009] U.S. Pat. Nos. 7,390,456, 8,173,063 and 6,316,136 and U.S.
Patent Application Publication No. 2011/0135531 describe various
interconnectors and methods of manufacturing interconnectors.
[0010] Powder metallurgy (PM) manufacturing methods have been used
to manufacture interconnectors due to PM's available net shape
forming capability. However, the components produced can contain
residual internal porosity which poses problems with associated
manufacturing methods and with final component function.
SUMMARY OF EMBODIMENTS OF THE INVENTION
[0011] The presence of the chromium nitrides (CrN's) in an
interconnector tends to be undesirable for two reasons. First, the
formation of the nitrides may cause a dimensional change to
interconnectors. Excessive nitride formation may lead to warping of
the interconnectors beyond allowable product dimensional tolerances
and hence can reduce manufacturing yield. Second, even though lower
levels of nitride may not have a significant effect on manufactured
dimensions, even lower levels of nitrides yet may be undesirable
with respect to SOFC function. In normal SOFC operation, the
interconnectors are exposed to elevated temperatures and air for
extended periods of time. In such an environment, nitrides
originally present within the interconnector material may grow and
introduce dimensional changes to the interconnector in-situ during
operation of the SOFC. Such dimensional changes may impair the
contact uniformity within the SOFC stack and hence lead to
accelerated degradation of electrical efficiency over time of
operation of the SOFC.
[0012] One or more embodiments of the present invention provide an
oxidation process for porous chromium components (e.g., PM
components such as interconnectors) that reduces the formation of
nitrides in the component.
[0013] One or more embodiments of the present invention provide a
method of oxidizing a porous component comprising at least 20
weight % chromium. The method includes: oxidizing the component in
a furnace so as to expose the component to an oxidation temperature
range for a predetermined time period; and during said oxidizing,
feeding a controlled atmosphere into the furnace. The controlled
atmosphere comprises at least 30 volume % nitrogen, at least 10
volume % oxygen, and at least 10 volume % water vapor. The
oxidizing increases a nitrogen content of the porous component by
less than 0.1 weight %.
[0014] According to one or more of these embodiments, after said
oxidizing, the component comprises less than 0.3, 0.2, 0.15, and/or
0.10 weight % nitrogen.
[0015] According to one or more of these embodiments, the
controlled atmosphere comprises at least 50 volume % ambient
air.
[0016] According to one or more of these embodiments, the
controlled atmosphere comprises at least 20 volume % water
vapor.
[0017] According to one or more of these embodiments, the
controlled atmosphere comprises between 10 and 30 volume % water
vapor.
[0018] According to one or more of these embodiments, the method
also includes adding water vapor to ambient air to create the
controlled atmosphere.
[0019] According to one or more of these embodiments, the oxidation
temperature range is above 750.degree. C. and the predetermined
time period is at least 5 hours.
[0020] According to one or more of these embodiments, the method
also includes feeding the component through the furnace in a travel
direction during said oxidizing, wherein the controlled atmosphere
is fed into the furnace in the travel direction.
[0021] According to one or more of these embodiments, the method
also includes feeding the component through the furnace in a travel
direction during said oxidizing, wherein the controlled atmosphere
is fed into the furnace in an opposite direction as the travel
direction.
[0022] According to various embodiments, the component and
controlled atmosphere may be fed through the furnace in the
oxidation step in concurrent or counter flow directions.
[0023] According to one or more of these embodiments, the component
comprises an SOFC interconnector.
[0024] Conventional wisdom in the interconnector industry was that
PM interconnector density should be maximized in order to obtain
maximum air/fuel impermeability. Because coarser iron particles are
more compressible, the industry has conventionally relied on such
coarser iron particles in an effort to maximize interconnector
density, and thereby maximize air/fuel impermeability. In contrast,
the present inventors discovered that according to various
embodiments of the invention, good impermeability could be achieved
at lower densities through the use of finer iron particles. It is
believed that the use of finer iron particles results in an
interconnector microstructure that is more easily sealed through
oxidation than the microstructure that results from a denser
interconnector made from coarser iron particles. According to
various embodiments, the ability to achieve good impermeability at
lower interconnector densities using finer iron particle sizes
enables less expensive manufacturing techniques (e.g., avoiding a
more expensive double-press procedure, using reduced sintering
temperatures and/or sintering times because smaller iron particle
size enhances chromium-to-iron diffusion which more readily
achieves in a target coefficient of thermal expansion (CTE)) and
reduces material cost by using less chromium per interconnector.
According to one or more embodiments, the reduced chromium content
requirement is advantageous because chromium is expensive, and
interconnectors comprise a major fraction of the SOFC hardware
cost. Reducing the total mass of the interconnectors may provide a
significant cost advantage.
[0025] One or more embodiments of the present invention provide a
faster, less expensive method for manufacturing an SOFC
interconnector with good impermeability and dimensional
characteristics.
[0026] One or more embodiments of the present invention provide an
SOFC interconnector that utilizes a reduced amount of chromium per
interconnector, thereby reducing the interconnector's material
cost.
[0027] One or more embodiments of the present invention provide a
Powder Metal (PM) process that enables fabrication of SOFC
interconnectors with a high chromium content (e.g., over 90%),
precise dimensional tolerances, thermal expansion properties that
match the thermal expansion properties of adjacent electrolytes,
and/or good impermeability. This combination is not readily
manufactured by other methods such as stamping or rolling. The PM
process according to one or more embodiments may provide a very
precise, cost effective fabrication of parts, to very precise
dimensional tolerances.
[0028] One or more embodiments of the present invention provide a
method of manufacturing an interconnector for a solid oxide fuel
cell. The method includes single-press compacting a powder blend to
form a green interconnector with a desired shape of a final
interconnector. The powder blend includes chromium and iron. At
least 50 wt % of an iron portion of the powder blend comprises iron
particles smaller than 45 um. The method also includes sintering
the single-pressed green interconnector to form a sintered
interconnector. The sintered interconnector comprises at least 90
wt % chromium and at least 3 wt % iron.
[0029] According to one or more of these embodiments, at least 60,
70, 80, and/or 90 wt % of the iron portion of the powder blend
comprises iron particles smaller than 60 um.
[0030] According to one or more of these embodiments, at least 60,
70, 80, and/or 90 wt % of the iron portion of the powder blend
comprises iron particles smaller than 45 um.
[0031] According to one or more of these embodiments, at least 40,
50, 60, 70, 80, and/or 90 wt % of the iron portion of the powder
blend comprises iron particles smaller than 30 um.
[0032] According to one or more of these embodiments, at least 30,
40, 50, 60, 70, 80, and/or 90 wt % of the iron portion of the
powder blend comprises iron particles smaller than 20 um.
[0033] According to one or more of these embodiments, the sintered
interconnector comprises between 94.5 and 95.5 wt % chromium and
between 4.5 and 5.5 wt % iron.
[0034] According to one or more of these embodiments, the method
also includes blending iron powder and an organic lubricant to form
a master iron/lubricant blend. The lubricant comprises at least 5
wt % of the master iron/lubricant blend. The method also includes
blending the master iron/lubricant blend with chromium powder to
form the powder blend, and delubricating the green interconnector
before said sintering.
[0035] According to one or more of these embodiments, the lubricant
comprises at least 1, 5, 10, and/or 20 wt % of the master
iron/lubricant blend.
[0036] According to one or more of these embodiments, the sintering
occurs over a sintering cycle time at a sintering temperature range
that does not exceed 1450.degree. C., 1425.degree. C., and/or
1400.degree. C.
[0037] According to one or more of these embodiments, the sintering
temperature range does not fall below 1150.degree. C., and the
sintering cycle time is less than 3, 2, and/or 1.5 hours.
[0038] According to one or more of these embodiments, the sintering
results in at least 70% and/or 80% diffusion of the chromium into
the iron.
[0039] According to one or more of these embodiments, the method
also includes oxidizing the sintered interconnector to form a final
interconnector, wherein the final interconnector is impermeable to
air and SOFC fuel. According to one or more of these embodiments,
the oxidizing comprises passing the sintered interconnector through
a continuous flow furnace in an interconnector travel direction
while feeding an oxygen containing gas into the furnace in the
interconnector travel direction. According to one or more of these
embodiments, the final interconnector comprises a flow field over
which air or gas is designed to flow during use of the
interconnector, the flow field is impermeable to SOFC fuel and air,
and the final interconnector has an average density within the flow
field of less than 6.8, 6.75, and/or 6.73 glee.
[0040] According to one or more of these embodiments, the green
interconnector comprises a flow field over which air or gas is
designed to flow during use of the interconnector, and the green
interconnector has an average density within the flow field of less
than 6.75, 6.73, and/or 6.70 g/cc.
[0041] One or more embodiments of the present invention provide an
interconnector for a solid oxide fuel cell. The interconnector
includes a sintered body comprising at least 90 wt % chromium and
at least 3 wt % iron. The body defines a flow field over which air
or gas is designed to flow during use of the interconnector. An
average density within the flow field is less than 6.75 g/cc. The
flow field is impermeable to SOFC fuel and air.
[0042] According to one or more of these embodiments, the
interconnector is formed from a pressed powder blend in which at
least 50 wt % of an iron portion of the powder blend comprised iron
particles smaller than 45 um.
[0043] According to one or more of these embodiments, the
interconnector is manufactured according to any one of the methods
disclosed herein.
[0044] One or more embodiments of the present invention provide an
interconnector for a solid oxide fuel cell. The interconnector
includes a sintered body comprising at least 90 wt % chromium and
at least 3 wt % iron. The interconnector is formed from a pressed
powder blend in which at least 50 wt % of an iron portion of the
powder blend comprised iron particles smaller than 45 um.
[0045] These and other aspects of various embodiments of the
present invention, as well as the methods of operation and
functions of the related elements of structure and the combination
of parts and economies of manufacture, will become more apparent
upon consideration of the following description and the appended
claims with reference to the accompanying drawings, all of which
form a part of this specification, wherein like reference numerals
designate corresponding parts in the various figures. In one
embodiment of the invention, the structural components illustrated
herein are drawn to scale. It is to be expressly understood,
however, that the drawings are for the purpose of illustration and
description only and are not intended as a definition of the limits
of the invention. In addition, it should be appreciated that
structural features shown or described in any one embodiment herein
can be used in other embodiments as well. As used in the
specification and in the claims, the singular form of "a", "an",
and "the" include plural referents unless the context clearly
dictates otherwise.
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] For a better understanding of embodiments of the present
invention as well as other objects and further features thereof,
reference is made to the following description which is to be used
in conjunction with the accompanying drawings, where:
[0047] FIG. 1 is a diagrammatic cross-sectional view of an SOFC
stack according to an embodiment of the invention;
[0048] FIG. 2 is a partial cross-sectional view of the SOFC stack
of FIG. 1;
[0049] FIG. 3 is a partial cross-sectional view of an
interconnector of the SOFC stack of FIG. 1;
[0050] FIG. 4 is a flowchart illustrating the manufacture of the
interconnector of FIG. 3 according to various embodiments of the
invention;
[0051] FIG. 5 is a plan view of the interconnector of the SOFC
stack of FIG. 1;
[0052] FIG. 6 illustrates the nature of formation of chromium
nitrides in a porous chromium alloy;
[0053] FIG. 7 shows temperatures, times and atmospheres used in the
oxidation process according to an embodiment of the present
invention;
[0054] FIG. 8 shows the effect of oxidation atmosphere on final
nitrogen content according to an embodiment of the present
invention; and
[0055] FIG. 9 is a diagram showing the oxidation of interconnectors
in an oxidation furnace according to one or more embodiments of the
present invention.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION
[0056] FIGS. 1 and 2 illustrate an SOFC stack 10 according to an
embodiment of the present invention. The SOFC stack 10 includes a
plurality of SOFCs 15. Each SOFC 15 includes an electrolyte plate
20 sandwiched between two electrodes (an anode plate 30 and a
cathode plate 40). A fuel side passage 50 (i.e., a series of
channels) for the passage of fuel 60 is disposed adjacent each
anode plate 30. An air side passage 70 (a series of channels) for
the passage of air 80 is disposed adjacent each cathode plate 40.
An interconnector 100 separates the fuel side passage(s) 50 for one
SOFC 15 from the air side passage(s) 70 of an adjacent SOFC 15.
[0057] The interconnector 100 may have any shape and size suitable
for use in an SOFC stack. In the embodiment illustrated in FIGS. 2
and 5, each side of the interconnector 100 includes a series of
alternating ridges 110 and valleys 120. As shown in FIG. 2, the
ridges 110 on opposite sides of the interconnector 100 abut the
electrodes 30,40, respectively, of adjacent SOFCs 15 such that the
spaces formed between the ridges 110, valleys 120, and respective
electrodes 30,40 create the fuel and air side passages 50, 70,
respectively.
[0058] FIG. 5 is a plan view of the fuel side of the interconnector
100. The fuel side passage 50 is defined by a depression 130 in the
interconnector 100. The depression 130 defines the valleys 120, and
the ridges 110 rise up from the depression 130. Fuel supply and
exhaust plenum regions 140, 150 are defined on upstream and
downstream sides of the ridges 110/valleys 120, respectively. A
fuel supply hole 160 leads into the fuel supply plenum 140. A fuel
exhaust opening 170 leads from the exhaust plenum region 150. Fuel
60 flows into the fuel side passage 50 and supply plenum 140 from
the supply opening 160, through the valleys 120 into the exhaust
plenum 150 (along with produced water), and out of the exhaust
opening 170.
[0059] Corresponding air side depression 130', valleys 120, ridges
110, and air supply and exhaust plenums 140', 150' and holes 160',
170' are disposed on the opposite side of the interconnector 100
and are shown in phantom dotted lines in FIG. 5.
[0060] The interconnector 100 includes a flow field that
encompasses the regions of the interconnector 100 over which fuel
or air are designed to flow. In the interconnector 100 illustrated
in FIG. 5, the flow field of the interconnector 100 is bounded by
the perimeter of the depressions 130, 130', and is generally +
shaped. The perimeter of the interconnector 100 outside of the
depressions 130, 130' are not part of the flow field. In
embodiments where the air/fuel passages 50, 70 extend beyond the
edges of the interconnector (i.e., to the top, bottom, left, and/or
right of the interconnector as viewed in FIG. 5), the flow field of
the interconnector extends to that edge. As explained in greater
detail below, it is typically important that the flow field portion
of the interconnector 100 be impermeable to fuel 60 and air 80.
[0061] In the embodiment illustrated in FIGS. 2 and 5, the ridges
110 and valleys 120 on one side of the interconnector 100 extend
perpendicularly relative to the ridges 110 and valleys 120 on the
other side of the interconnector 100. As a result, as illustrated
in FIG. 2, the fuel side passages 50 extend into the sheet and the
air side passages 70 extend left to right. Consequently, the SOFC
stack 10 is designed so that the fuel 60 flows in one direction,
while the air 80 flows in a perpendicular direction. However,
according to alternative embodiments, the fuel and air side
passages 50,70 may be parallel (e.g., as shown in the alternative
interconnector 100' illustrated in FIG. 3) or run in any other
suitable direction relative to each other without deviating from
the scope of the present invention.
[0062] Hereinafter, methods of making the interconnector 100
according to various embodiments are described with reference to
FIG. 4.
[0063] Chromium (Cr) base powder 200 is produced from coarse
chromium feedstock of about 20 mm to 6 mm.times. down by grinding
with hammer mills, pin mills, and/or other suitable grinding
machinery and then classified. The coarse chromium feedstock
according to various embodiments comprises at least 90%, 95%, 97%,
98%, 99%, and/or 99.3% chromium (e.g., aluminothermic chromium,
chromium powder produced using another suitable method).
[0064] Unless otherwise stated, all percentages disclosed herein
are weight percentages. Unless otherwise stated particle sizes
refer to screen classification using square openings. For example,
particles smaller than 45 um mean particles that fall through a 45
um.times.45 um square opening. In contrast any dXX values (e.g.,
D50) refer to the XX % distribution particle by number of particles
(not by weight). Thus, a powder with a D50 of 100 um means that 50%
of the particles (by number of particles, not mass) are larger than
100 um and 50% are smaller.
[0065] According to various embodiments, the chromium powder is
classified to under 160 um (i.e., substantially all particles fall
through a 160 um.times.160 um opening) via a suitable screen, with
a D50 of somewhere between 80-150 um and/or between 110-150 um, and
a maximum of 5%, 10%, 20%, and/or 30% chromium particles smaller
than 45 um to create the chromium base powder 200. According to
various embodiments, the chromium base powder 200 comprises no more
than 5% chromium particles larger than 200 um, no more than 10%
chromium particles larger than 160 um, as much as 100% chromium
particles larger than 63 um, and no more than 1% chromium particles
smaller than 45 um. According to various other embodiments, the
chromium base powder 200 comprises no more than 1% chromium
particles larger than 160 um, at least 75% chromium particles
larger than 63 um, and no more than 15% chromium particles smaller
than 45 um. According various embodiments, the chromium base powder
200 comprises no more than 0.1% chromium particles larger than 200
um, no more than 2% chromium particles larger than 160 um, 80-100%
and/or 84-96% chromium particles larger than 63 um, and no more
than 5% chromium particles smaller than 45 um.
[0066] Iron (Fe) powder 220 is blended with a lubricant (e.g., an
organic lubricant, an organo-metallic lubricant, or any other type
of suitable lubricant that can be used in pressed PM) 230 to create
a master iron/lubricant blend 240. According to various
embodiments, the iron powder 220 comprises at least 95%, 97%, 98%,
99%, 99.5%, and/or 99.9% pure iron. According to various
embodiments, the Iron powder 220 comprises at least 30%, 40%, 50%,
60%, 70%, 80%, 90%, 95%, 97%, 98%, 99%, 99.5%, 99.6%, 99.7%, 99.8%,
and/or 99.9% iron particles smaller than 75, 70, 60, 50, 45, 40,
35, 30, 25, 20, 15, and/or 10 um. According to various embodiments,
the iron powder 220 may comprise any combination of these
percentages and size limitations (e.g., anywhere from 30% being
smaller than 75 um to 99.9% being smaller than 10 um).
[0067] The iron powder 220 may be heterogeneous such that, for
example, at least 90% of iron particles are smaller than 50 um and
at least 50% are smaller than 20 um. Again, any combination of sets
of the above-listed percentages and size limits may be used. A
combination of coarser and finer iron particles may be used to
provide the better flow and compression characteristics of larger
iron particles, while still providing the improved impermeability
characteristics of smaller iron particles.
[0068] According to one or more embodiments, the iron powder 220
comprises a high purity, fine iron powder such as a powder having a
typical screen analysis of d10 5 um, d50 15 microns, and d90 30
microns and a chemical analysis (wt %) of 98+% iron, 0.150% carbon,
0.800% oxygen, 0.015% sulphur, and 0.010% phosphorus, or a powder
having a typical chemical analysis of 99.7% iron, 99.5% iron-met,
0.09% O-tot, 0.003% C, 0.009% S, 0.005% P, 0.002% Si, 0.09% Mn and
a typical sieve analysis of 0.0% over 150 microns, 0.3% between
75-150 microns, 1% 63-75 microns, 12% 45-63 microns, and 87% under
45 microns, or a mixture of such powders (e.g., 75/25, 50/50,
25/75).
[0069] According to various embodiments, the master iron/lubricant
blend 240 has an organic lubricant 230 weight percentage of between
1 and 30%, between 5 and 25%, between 10 and 20%, and/or between
12.5 and 17.5%. Embodiments using iron powders 220 with smaller
particles sizes may be first separately combined with larger
amounts of lubricant than in embodiments in with coarser iron
powders because ease of flow tends to be inversely proportional to
particle size. However, according to various embodiments, lubricant
230 is omitted altogether. For example, one or more embodiments
using coarser iron powder may not use any lubricant 230.
[0070] The chromium base powder 200 and master iron/lubricant blend
240 are then blended to create a final blend powder 260. According
to various embodiments, the final blend powder 260 comprises at
least 90, 91, 92, 93, 94, and/or 95% base chromium powder. The
balance of the final blend powder 260 preferably comprises the
master iron/lubricant blend 240. According to various embodiments,
the final blend powder 260 comprises at least 0.4% organic
lubricant 230. According to various embodiments, the final blend
powder 260 comprises between 1 and 9% iron. According to one or
more embodiments, the final blend powder 260 comprises about 94-96%
chromium, at least 4% and/or 5% iron, and at least 0.10%, 0.2%,
0.3%, and/or 0.4% lubricant 230. According to one or more
embodiments, the final blend powder 260 comprises 0.65% organic
lubricant.
[0071] According to various embodiments, the chromium base powder
200 and master iron/lubricant blend 240 are blended at about room
temperature (e.g., between 15.degree. C. and 27.degree. C. and/or
about 21.degree. C.) to form the final blend powder 260. According
to one embodiment, a double cone blender and 40 minute blending
cycle is used.
[0072] According to other embodiments, the chromium base powder 200
and master iron/lubricant blend 240 are blended at temperatures
above room temperature (e.g., above 27, 40, 50, 70, and/or 100, and
below 140.degree. C., 130.degree. C., 120.degree. C., and/or
110.degree. C.) to form the final blend powder 260. According to
one embodiment, a jacketed DC blender and a 2 hour cycle (including
heating time and blending time) is used. According to one or more
embodiments, blending at elevated temperatures proves good flow
characteristics. According to various embodiments, the blending
temperature is kept below a inciting temperature of the lubricant
230.
[0073] A die-cavity having the desired cavity shape of the final
interconnector 100 is then appropriately filled with the final
blend powder 260. After the die-cavity is filled with the final
blend powder 260, the final blend powder 260 is single-stage
compacted/pressed in a closed die to form a green interconnector
280. According to various embodiments, the green interconnector 280
has essentially the final shape and size of the final
interconnector 100 (except for minor size and shape changes that
result from post-pressing elastic rebound, sintering, further heat
treatments, and/or oxidation). According to various embodiments,
the single stage compaction creates the ridges 110, valleys 120,
depressions 130, 130', plenums 140, 140', 150, 150', and holes 160,
160', 170, 170'. According to various embodiments, the compaction
is carried out at 40-100 Tsi and/or 60-75 Tsi using a press (e.g.,
a hydraulic press, a hybrid press, or any other suitable
press).
[0074] According to various embodiments, the compaction/pressing is
carried out via a single pressing procedure, as opposed to a
conventional two-stage pressing procedure (e.g., the two-stage
pressing procedure disclosed in U.S. Pat. No. 8,173,063).
[0075] According to various embodiments, the green interconnector
280 has a green strength of at least 400, 500, 600, and/or 700 psi.
According to various embodiments, the green interconnector 280 has
an average green density within the flow field of at least 6.50,
6.55, 6.60, 6.63, 6.65, and/or 6.67 g/cc and/or less than 6.80,
6.78, 6.75, 6.72, 6.70, 6.68, 6.66, and/or 6.65. According to one
or more embodiments, the green density is about 6.65 g/cc on
average in the flow field.
[0076] According to one or more embodiments, if lubricant 230 was
used, the green interconnector 280 is delubricated in air at
between 300.degree. C. and 500.degree. C. (e.g., about 400.degree.
C.) for 1 to 3 hours to substantially remove the lubricant 230 and
form a delubricated green interconnector 300. However, depending on
the lubricant 230 properties and content and the size and
dimensions of the green interconnector 280, alternative
temperatures and/or delubricating times may be used.
[0077] The delubricated green interconnector 300 (or green
interconnector 280 if lubricant was not used) is then sintered to
form a sintered interconnector 320. According to various
embodiments, the delubricated green interconnector 300 is sintered
in a furnace maintained within a sintering temperature range (e.g.,
at temperatures that are at least 1150.degree. C. and/or
1250.degree. C. and are less than 1450.degree. C., 1425.degree. C.,
and/or 1400.degree. C.) over a sintering cycle time that is between
30 minutes and 3 hours, 45 minutes and 2 hours, and/or 1 and 11/2
hours to metallurgically bond the chromium and iron particles
together and diffuse the chromium into the iron. According to
various embodiments, the sintering cycle time is less than 3, 2,
and/or 1.5 hours at the sintering temperature range. According to
various embodiments, the sintering environment comprises at least
80%, at least 90%, and/or up to 100% H.sub.2. According to one or
more embodiments, the delubricated green interconnector is sintered
for a cycle time of 70 minutes in a furnace with a sintering
temperature that ranges from 1150.degree. C. to 1380.degree. C.
over the course of the 70 minute cycle in a sintering environment
that comprises about 95% H.sub.2 and about 5% Ar. According to one
or more embodiments, sintering is carried out in a pusher furnace
with two sealed exit doors and at least 5 zones of thermal
control.
[0078] For a given chemistry interconnector (e.g., 95% chromium/5%
iron), coarser iron particles result in fewer chromium/iron contact
points through which diffusion can occur. Coarser iron particles
also result in longer pathways into the center of each iron
particle. The fewer contact points and longer pathways typically
require high sintering temperatures (e.g., over 1450.degree. C.)
and/or longer sintering times to achieve the desired diffusion
levels and associated target coefficient of thermal expansion (CTE)
levels. Higher sinter temperatures and longer processing times tend
to result in higher manufacturing costs. In contrast, the use of
smaller iron particle sizes according to various embodiments
facilitates lower sintering temperatures and sintering times while
still achieving desired diffusion/CTE levels.
[0079] According to various embodiments, the atmosphere flow in the
sintering furnace reduces the surface iron and chromium oxides,
which are barriers to diffusion, allowing particle bonding and
diffusion to proceed.
[0080] According to various embodiments, a thermal profile of the
sintering step results in a level of chromium into iron diffusion
of at least 60%, 70%, 75%, 78%, and/or 80% throughout the sintered
interconnector 320. According to various embodiments, the sintering
results in 80-85% diffusion of the chromium into the iron.
[0081] According to various embodiments, the sintered
interconnector 320 has an average density in the flow field of at
least 6.50, 6.55, 6.60, 6.63, 6.65, 6.67, 6.68, 6.69, and/or 6.70
g/cc, and/or less than 6.8, 6.78, 6.75, 6.73, 6.70, and/or 6.68
g/cc. According to one or more embodiments, the sintered density is
about 6.65 g/cc on average in the flow field. According to various
embodiments, some densification is achieved through sintering
(e.g., a 0.5-2% density increase from the green interconnector
density).
[0082] According to various embodiments, the sintering process
results in a sintered interconnector 320 with a nitrogen content of
less than 0.10%, 0.09%, 0.08%, 0.07%, and/or 0.065%. According to
various embodiments, the low nitrogen content may prevent or limit
distortion of the final interconnector 100. According to various
embodiments, nitrogen content of the interconnector is reduced by
reducing the nitrogen content of the atmosphere to which the
interconnector is exposed (e.g., before, during, or after
sintering).
[0083] Because SOFCs experience a wide temperature range during use
(e.g., from startup, through operation, and then through shutdown),
it is typically preferable for the final interconnector 100 to have
a coefficient of thermal expansion (CTE) that is about equal to the
CTE of the electrolyte plate 20 so that they synchronously expand
and contract during startup, operation, and shutdown of the SOFC
stack 10. According to various embodiments, the combination of
chromium/iron ratio and sintering protocol (which controls the
resulting degree of chromium-into-iron diffusion) impact the
resulting CTE of the final interconnector 100. Consequently, the
chromium/iron ratio and sintering protocol may be tailored to match
the CTE of the final interconnector 100 with the CTE of
electrolytes commonly used in SOFCs. According to one or more
embodiments, an interconnector 100 with a 95% chromium/5% iron
content and over 80% diffusion has a CTE that is well suited to one
or more commonly used types of electrolyte plates 20.
[0084] According to various embodiments, the sintered
interconnector 320 is thermally stabilized and sealed by oxidation
at oxidation temperatures of between 500.degree. C. and
1100.degree. C. (e.g., at least 500.degree. C., 600.degree. C.,
700.degree. C., 800.degree. C., and/or 900.degree. C., and/or
between 900.degree. C. and 1000.degree. C., and/or less than
1200.degree. C., 1100.degree. C. and/or 1000.degree. C.) for at
least 5, 10, 15, and/or 20 hours and less than 40, 35, 30, and/or
25 hours. According to one or more embodiments, oxidation begins to
take place at a reasonably fast rate at temperatures of 500.degree.
C. and above. According to one or more embodiments, oxidation is
carried out by keeping the sintered interconnector 320 in a
950.degree. C. oxidation environment for 20-24 hours. FIG. 7
illustrates an oxidation process according to one or more
embodiments, in which the furnace atmosphere to which the
interconnectors are exposed ramps from ambient temperature (e.g.,
25.degree. C.) to 950.degree. C. over 5 hours. The environment is
maintained at about 950.degree. C. for 24 hours. The environment is
then ramped back down to ambient temperature over about 7
hours.
[0085] As shown in FIG. 9, according to various embodiments,
sintered interconnectors 320 are oxidized in a continuous process
in which the sintered interconnectors 320 are stacked on a furnace
mesh belt 500 on ceramic setters that may help to maintain flatness
of the resulting final interconnectors 100. A controlled atmosphere
510 (described in greater detail below) is fed into the oxidizing
furnace 520 in the direction that the mesh belt 500 and sintered
interconnectors 320 flow to provide the reaction gas (oxygen) to
the environment around the interconnectors 320 within the furnace
520. According to various embodiments, such concurrent flow
direction may help facilitate oxidation as the interconnectors 320
heat up (e.g., between 500.degree. C. and 700.degree. C.) and
before nitridation might otherwise take over at higher temperatures
(e.g., at or above 700.degree. C.). Such concurrent flow may
additionally or alternatively improve the oxidation cycle by
moderating the temperatures to which the interconnectors are
exposed (e.g., perhaps by causing the temperatures experienced by
the interconnectors in the beginning of the oxidation cycle to ramp
up more slowly and/or uniformly). According to alternative
embodiments, the interconnectors 320 may be oxidized in a batch
furnace instead of a continuous flow furnace. The controlled
atmosphere 510 may be fed through the batch furnace over the course
of the oxidation batch process to maintain an available supply of
oxygen for the oxidation process.
[0086] According to various alternative embodiments, the controlled
atmosphere 510 may be provided to the furnace 520 in a counter flow
direction, rather than a concurrent flow direction. In various
counter flow embodiments, the controlled atmosphere enters the
furnace 520 at or around the portion of the furnace 520 where the
oxidized interconnectors 100 exit, and exhausts out of the furnace
520 at or around the portion of the furnace where the sintered
interconnectors 320 enter the furnace 520). This alternative
counter flow process is similar to the process shown in FIG. 9, but
with arrows 510 and 580 shown in flipped directions and positions,
and the humidifier 560 being repositioned accordingly.
[0087] According to various embodiments, the controlled atmosphere
510 is fed into the furnace 520 continuously throughout the entire
oxidation cycle starting as soon as the interconnectors 320 are
initially fed into the furnace 520. According to alternative
embodiments, the controlled atmosphere 510 is only fed into the
furnace 520 while the interconnectors 320 are exposed to an
oxidizing temperature environment (e.g., when the interconnectors
are exposed to an environment with a temperature that is above
300.degree. C., 400.degree. C., and/or 500.degree. C.).
[0088] As shown in FIG. 6, when oxidizing sintered interconnectors
320 in ambient air (e.g., air with about 1-4% water vapor content)
it has been observed that nitrides of chromium can form within the
internal microstructure. After oxidation of a porous Cr alloy
interconnector in ambient air the microstructure tends to show
areas of enrichment of nitrogen in the areas surrounding the pores
and also within the inner material grain boundaries as shown in
FIG. 6 after exposure to the oxidation process in ambient air.
[0089] The formation of nitrides is a result of the combination at
elevated temperature of the Cr base metal and nitrogen contained in
the ambient air. According to various embodiments, the amount of
such nitrides in the interconnector is preferably reduced. Thus,
one or more embodiments of the present invention provide an
oxidation process for porous chromium components (e.g., PM
components such as interconnectors) that reduces and/or minimizes
the formation of nitrides in the component. Reducing the extent of
nitride formation in the interconnector may increase the overall
interconnector yield during manufacturing (e.g., because more of
the interconnectors 100 are within dimensional tolerances) and may
result in interconnectors with improved life-long dimensional
accuracies during use in an SOFC stack. Methods of reducing
nitrogen absorption have been suggested in the literature for fully
dense Cr materials, for example Michalik used a mixture of (1)
nitrogen with 4% H.sub.2O, and 4% H.sub.2, or (2) nitrogen with 10%
H.sub.2O to suppress nitride formation. However, as shown in FIG.
8, those methods were found to be ineffective when applied to
porous Cr alloys (e.g., PM interconnectors) where nitrogen content
after oxidation actually increased to in excess of 1 wt %. See
Michalik 2007 Effect of water vapour on growth and adherence of
chromia scales, Julich Research Thesis.
[0090] As shown in FIG. 8, according to one or more embodiments,
oxidation in a nitrogen-free Argon/Oxygen mixture may maintain
nitrogen content to the pre-oxidized level of around 0.05%.
Accordingly, various embodiments of the present invention utilize a
substantially nitrogen-free Ar/O atmosphere during the oxidation
process. However, according to various embodiments, the use of an
Ar/O atmosphere is not practical due to high cost of process
atmosphere or the need to use complex and costly manufacturing
equipment with atmosphere recycling capability.
[0091] According to one or more alternative embodiments, the
interconnectors 320 are oxidized in a controlled atmosphere 510
comprising ambient air 540 and an elevated level of water-vapor
550. According to various embodiments, as shown in FIG. 9, the
controlled atmosphere 510 is created by humidifying ambient air 540
in a humidifier 560 to create the controlled atmosphere 510.
According to various embodiments, the water vapor content of the
controlled atmosphere 510 that is pumped into the furnace 520
during the oxidation step comprises ambient air 540 with a water
vapor content (by volume) of at least 5%, 10%, 15%, 20%, and/or
25%, less than 50%, 40%, and/or 35%, and or between 10% and 40%,
between 10% and 30%, and/or between 15% and 25%. According to one
or more embodiments the water vapor content in the controlled
atmosphere is 20%. According to one or more of these embodiments,
this controlled atmosphere 510 has been found to provide an
effective means of controlling/limiting the final nitrogen content
in the oxidized Cr alloy. According to various embodiments, the
ambient air 540 to which the water vapor 550 is added already
includes (by volume %): [0092] 60-95%, 70-90%, 70-85%, 75-85%,
and/or about 78% nitrogen (N); [0093] 5-35%, 10-30%, 15-25%, and/or
about 21% oxygen (O.sub.2); and [0094] 0-4% water vapor
(H.sub.2O).
[0095] The amount of water vapor 550 to be added to the ambient air
540 will depend on the starting humidity of the ambient air 540.
According to various embodiments, less water vapor 550 is added to
more humid air 540 to create the controlled atmosphere.
[0096] According to various embodiments the controlled atmosphere
510 comprises: [0097] 30-95%, 40-90%, 45-80%, 45-70%, 50-60%,
and/or about 55% nitrogen (N); [0098] 5-40%, 5-35%, 10-30%, 10-25%,
10-20% and/or about 15% oxygen (O.sub.2); and [0099] 5-50%, 10-40%,
10-35%, 20-35%, and/or about 30% water vapor (H.sub.2O).
[0100] Unless otherwise specifically stated, all atmospheric
percentages are volume percentages based on the atmosphere being at
standard ambient temperature and pressure (SATP) (i.e., 25.degree.
C. and 101.3 kPa). All atmospheric percentages may alternatively be
considered to be molar percentages at SATP. Thus, according to
various embodiments, the controlled atmosphere comprises 5-50%,
10-40%, 10-35%, 20-35%, and/or about 30% water vapor (H.sub.2O) by
volume and/or by molar concentration. According to various
embodiments, the controlled atmosphere 510 being injected into the
furnace 520 is actually injected at approximately SATP, such that
the volume percentages may be measured as they are injected into
the furnace 520. According to alternative embodiments, the
controlled atmosphere 510 may be injected into the furnace 520 at
other temperatures or pressures (though the atmospheric percentages
are still measured at SATP).
[0101] The water vapor content of the controlled atmosphere 510 may
alternatively be measured in terms of dew point. According to
various embodiments, the dew point of the controlled atmosphere 510
(at standard ambient pressure of 101.3 kPa) is at least 40.degree.
C., 45.degree. C., 50.degree. C., and/or 55.degree. C., and/or
between 40.degree. C. and 100.degree. C., between 45.degree. C. and
90.degree. C., between 45.degree. C. and 80.degree. C., between
50.degree. C. and 80.degree. C., between 55.degree. C. and
80.degree. C., and/or about 60.degree. C.
[0102] According to various embodiments, the ambient air 540 may be
altered in other ways in addition to having water vapor 550 added
to form the controlled atmosphere 510. For example, oxygen may also
be added to the ambient air 540 to form the controlled atmosphere
510. Added oxygen may increase the oxidation rate and allow a
reduction in the oxidation cycle time.
[0103] According to various embodiments, the nominal flow rate of
the controlled atmosphere 510 into the furnace 520 during the
oxidation process is 125 cubic feet per hour per inch of furnace
belt 500 width, with a minimum of 42 cubic feet per hour per inch
of furnace belt 500 width and a maximum of 208 cubic feet per hour
per inch of furnace belt 500 width. According to various
embodiments, the controlled atmosphere 510 is fed into the furnace
520 during the oxidation process at at least 25, 35, 40, 50, 60,
70, 80, 90, and/or 100 cubic feet per hour per inch of furnace belt
500 width, and/or between 25 and 500, between 25 and 400, between
40 and 250 cubic feet per hour per inch of furnace belt 500 width.
According to one or more embodiments, the furnace belt 500 is 18
inches wide. According to one or more embodiments, an array of 27
sintered interconnectors 320 are stacked 3.times.3.times.3 on
ground alumina setter plates and then oxidized to form the final
interconnectors 100. According to one or more alternative
embodiments, the sintered interconnectors 320 are stacked 5 high
and three across the mesh belt.
[0104] According to various embodiments, the desired humidification
is accomplished using a humidifier 560 with an 8 lb./hour capacity
to support a 1000 cubic feet per hour flow of the controlled
atmosphere 510 into the furnace. According to One or more
embodiments, the controlled atmosphere 510 is fed into the furnace
520 at a rate of at least 100, 250, 500, 750 cubic feet per hour
(cfh), and/or between 100 and 5000 cfh, between 500 and 4000 cfh,
and/or between 750 and 4000 cfh.
[0105] As shown in FIG. 9, after flowing into the furnace 520 and
providing reaction gas for the oxidation step, the used controlled
atmosphere 510 (less used reaction gas and other lost components)
is exhausted from the furnace 520 as exhaust gas 580 where the belt
500 exits the furnace 520. According to various embodiments, the
exhaust gas 580 may be recycled and re-injected (e.g., by
re-humidifying the exhaust gas 580 to form the controlled
atmosphere 510).
[0106] According to one or more embodiments, the manufacturing
process results in the final interconnector 100 having a nitrogen
content after the oxidation step of no more than 1.0%, 0.75%, 0.5%,
0.4%, 0.3%, 0.20%, 0.17%, 0.15%, 0.12%, 0.10%, and/or 0.09%. As
shown in FIG. 8, after oxidation in the controlled atmosphere
containing water vapour, the resulting nitrogen content is
substantially reduced relative to the values observed after
oxidation in ambient air (although ambient air may alternatively be
used according to various embodiments). The measured nitrogen
content is similar to that seen when oxidized in the nitrogen free
argon/oxygen atmosphere. According to various embodiments, the
oxidation process increases the nitrogen content of the
interconnector by less than 0.1, 0.09, 0.08, 0.07, 0.06, 0.05
and/or 0.00 wt % of the final interconnector 100.
[0107] According to various embodiments, the oxidation step results
in the formation of an oxide layer on the surface of the
interconnector, wherein the oxide (e.g., chromium oxide,
Cr.sub.2O.sub.3) is at least 1, 2, and/or 3 um thick and/or between
3 and 4 um thick.
[0108] According to various embodiments, the oxidizing step results
in the final interconnector 100. According to various embodiments,
the final interconnector 100 has an average flow field density of
at least 6.63, 6.65, 6.67, 6.68, 6.69, 6.70, 6.71, 6.72, 6.73,
and/or 6.71 glee, and/or less than 6.8, 6.78, 6.75, 6.74, 6.73,
6.72, and/or 6.71 g/cc. According to one or more embodiments, the
final interconnector 100 has an average density within the flow
field of about 6.7 glee. According to one or more embodiments, the
final interconnector 100 is flat to within 400, 350, and/or 300
microns. According to various embodiments, an overall thickness of
the interconnector plate 100 is between 1.5 min and 3.5 mm
(depending on the embodiment), with a thickness variation of 0.25,
0.20, 0.19, and/or 0.180 microns maximum (not including in the
depressions 130, 130').
[0109] According to various embodiments, the final interconnector
100 is subjected to further manufacturing steps (e.g., coatings,
etc.) before being used in the SOFC stack 10.
[0110] While the above oxidation process is described with respect
to particular interconnectors, the oxidation process may
additionally or alternatively be used on a wide variety of other
components without deviating from the scope of the present
invention. For example, the above described oxidation process may
be used with interconnectors made using other manufacturing
techniques (e.g., interconnectors made using double-press
manufacturing techniques). The oxidation process according to one
or more embodiments of the present invention may be used to
oxidize/passivate porous PM components (e.g., high chromium content
PM components).
[0111] Conversely, while the interconnector manufacturing process
is described as using various particular oxidation steps, the
manufacturing method and resulting interconnectors 100 may
alternatively be made using any other suitable steps (e.g.,
alternative oxidation steps, oxidation steps that utilize only
ambient air as the atmosphere, methods that omit a formal oxidation
step altogether, etc.).
[0112] According to various embodiments, iron particle size,
chromium particle size, density, surface oxidation, and/or other
aspects of the manufacturing process make the interconnector 100
impermeable to air from the cathode side 70 and fuel from the anode
side 50. According to various embodiments, the final interconnector
100 thereby provides the dimensional accuracy, impermeability, and
CTE that are suited for good function as an SOFC interconnector
100.
[0113] According to various embodiments, the final interconnector
100 consists essentially of chromium and iron. According to various
embodiments, chromium and iron comprise at least 99.0, 99.5, 99.7,
99.8, 99.8, 99.9, and/or 99.99% of the interconnector 100.
[0114] As used herein, the term "impermeable to SOFC fuel and air"
and similar terms means impermeability as that term is understood
in the SOFC interconnector art. SOFC interconnector impermeability
does not require absolute impermeability to fuel and air. Rather,
"impermeable" merely requires the interconnector to be sufficiently
impermeable to provide good function to an SOFC without failure
over an extended period of time.
[0115] While embodiments of the invention have been described above
with respect to SOFC interconnectors 100, embodiments of the
invention may also be applied to other types of components. Various
embodiments are particularly applicable to components in which a
high density and/or impermeability is desired and/or components
with complex finished shapes.
[0116] The foregoing illustrated embodiments are provided to
illustrate the structural and functional principles of embodiments
of the present invention and are not intended to be limiting. To
the contrary, the principles of the present invention are intended
to encompass any and all changes, alterations and/or substitutions
within the spirit and scope of the following claims.
* * * * *